Such an interband cascade laser amplifier medium is known, for example, from U.S. Pat. No. 5,799,026 or from US 2010/0097690 A1. In this case, the optical transition used for the laser activity takes place between the hole quantum film and the electron quantum film. In this case, quantum film means that on account of the thickness of the corresponding semiconductor layers and as a result of the localization of the electrons in the conduction band of the electron quantum film and of the holes in the hole quantum film on account of the band profile with respect to adjacent layers, there is a quantization of the population levels perpendicular to the layer plane. Through suitable choice of the semiconductor materials, in particular the valence band edge of the hole quantum film is energetically above the conduction band edge of the electron quantum film. For this case, the emission wavelength in the case of an optical transition of an electron from the conduction band of the electron quantum film into the valence band of the hole quantum film becomes practically independent of the respective band gaps of the semiconductor materials involved. This allows, for example, semiconductor laser emissions in a wavelength range of between 3 μm and 5 μm, which, in the case of a dependence of the optical transition on the band gap of the semiconductor materials, has not been possible in an uninterrupted fashion heretofore in continuous wave operation at room temperature. Laser emissions in the so-called medium infrared range (MWIR) between 2.5 μm and 8 μm are of interest particularly for chemical analyses, for target seeking devices or for applications in the medical field. The optical transition of the electron takes place between the conduction band of the electron quantum film and the valence band of the hole quantum film, that is to say between spatially adjacent semiconductor materials. A locally indirect band transition is involved. In the present case, this is referred to as a so-called type II semiconductor laser in this context. By contrast, if the optical transition takes place locally directly between the conduction band and the valence band of the same semiconductor material, then this is referred to as a type I semiconductor laser.
InAs, InAsSb, InGaAs or InAlAs are known as semiconductor materials for the electron quantum film from U.S. Pat. No. 5,799,026. GaSb, GaInSb, GaSbAs and GaAlSb are disclosed as semiconductor materials for the adjacent hole quantum film in U.S. Pat. No. 5,799,026. The position of the conduction band edges and of the valence band edges of the electron and hole quantum films is configured for optimizing the optical transition through the choice of the III-V compound semiconductors and the thickness of the quantum films.
In contrast to a semiconductor diode laser, such as is described in U.S. Pat. No. 5,793,787, for example, a unipolar transport of a single charge carrier type, that is to say either of electrons or of holes, along the laser material takes place in the case of an interband cascade laser. For this purpose, an external voltage is applied to the laser material, such that charge carriers of one type migrate into the laser material on one side and leave the laser material again on the other side. Accordingly, the entire semiconductor material of a laser of this type has a uniform charge carrier doping. An n-type doping is provided for the transport of electrons as uniform majority charge carriers; a p-type doping for the transport of holes as uniform majority charge carriers.
A different concept in turn is pursued by a so-called quantum cascade laser such as is described in U.S. Pat. No. 6,137,817, for example. In that case, too, a unipolar transport of charge carriers, in particular of electrons, along the laser material takes place. The laser material of the quantum cascade laser comprises strung-together amplifier regions and injector regions which are formed by electron quantum films separated from one another by means of barrier layers. Both the optical transition and the transport of the electrons to the next amplifier region take place exclusively in the conduction band of the laser material.
In order to be able to use an electron that has relaxed in the amplifier region of an interband cascade laser as a result of optical transition into the valence band of the hole quantum film for the purposes of a cascade multiply for further optical transitions, in an interband cascade laser amplifier medium the amplifier region is adjoined by an electron collector region and an electron injector region. The electron collector region comprises at least one collector quantum film separated by means of an electron barrier layer. The electron injector region comprises an injector quantum film separated by means of an electron barrier layer. The valence band edge of the collector quantum film, that is to say of the third semiconductor material, is energetically adapted for taking up an electron from the valence band of the hole quantum film, that is to say of the first material. The conduction band edge of the injector quantum film, that is to say of the fourth semiconductor material, is energetically adapted for taking up an electron from the valence band of the third semiconductor material.
According to U.S. Pat. No. 5,799,026 or US 2010/0097690 A1, a plurality of collector and injector quantum films and barrier layers can alternate both in the electron collector region and in the electron injector region. The barrier layer used between the amplifier region and the electron collector region prevents undesirable tunneling of the electron from the electronic level of the electron quantum films without the electron having relaxed radiatively into the energetically lower energy level in the hole quantum film.
In accordance with the prior art cited, the collector quantum films are configured with regard to their thickness and the choice of semiconductor material such that, for example, the highest quantized hole level therein corresponds energetically approximately to the highest quantized hole level in the valence band of the hole quantum film. In particular, in the operating case the approximately linear profile of the electric field resulting from the applied external voltage within the semiconductor material should be taken into consideration in this case. The electron is thus allowed in particular to tunnel resonantly from the valence band of the hole quantum film into the valence band of a collector quantum film.
In order to have the electron available again for an optical transition in a further cascade, the electron collector region of one cascade is adjoined by the electron injector region of the next cascade. The task of the electron injector region is to transfer the electron from the valence band of the collector quantum film into an electronic level in the conduction band of the injector quantum film, such that it can relax from there via the conduction band of an adjoining electron quantum film radiatively again into the valence band of a hole quantum film.
For this purpose, the conduction band of the adjoining injector quantum films is configured through the choice of the thickness and of the semiconductor material such that, by way of example, the lowest quantized level therein, taking the field profile into consideration, is energetically approximately identical to the highest quantized hole level in the valence band of the last collector quantum film.
GaSb, GaInSb or GaSbAs, inter alia, are known as semiconductor materials for the collector quantum film from U.S. Pat. No. 5,799,026. The materials of the electron quantum film are used as materials for the injector quantum film. On account of the field profile resulting from the external voltage in the operating case within the semiconductor material, which leads to band tilting, an electron when passing through the semiconductor material can thus be used multiply for the same optical transition at different locations. For this purpose, a plurality of cascade are connected in series by means of a corresponding layer construction. The electron collector region of one cascade takes up the electron that has undergone transition and passes it on to the electron injector region of the next cascade. The latter injects the electrons again into an amplifier region, where they relax as a result of optical transition. Overall, in this way an electron rapidly passing through the laser medium is utilized multiply for an optical transition.
The barrier layers enabling the electrons to tunnel by providing the potential barriers are constructed, in accordance with U.S. Pat. No. 5,799,026, in particular from semiconductor materials such as AlSb, AlInSb, AlSbAs or AlGaSb. These materials have a relatively large band gap. The levels—relevant to the optical transition—of the electrons and holes in the conduction band and in the valence band of the adjacent layers are energetically within the band gap of the barrier layers.
In the case of a diode laser, by contrast, the optical transition is situated within the depletion zone of a p-n junction. Charge transport predominantly takes place by means of electrons in the n-doped region, and by means of holes in the p-doped region. For laser operation, the radiative recombination of electrons and holes in the depletion zone is crucial, which are injected from opposite directions (contacts). In other words, bipolar charge transport by means of electrons and holes takes place. Electrons that have relaxed radiatively in the amplifier region are not transported further. Consequently, each injected electron in the case of a diode laser can contribute maximally to the emission of one photon in the device.
For the electrical connection of the laser material of an interband cascade laser, specific connection and termination layers are furthermore known from the prior art. However, the exact construction of these layers is not the subject matter of the present invention.
The forwarding of the electrons after their optical transition from the valence band of the hole quantum film into the electron collector region, and from there into the adjoining electron injector region, which passes the charge carriers once again into the electron quantum film of the next amplifier region, takes place by means of tunneling processes through the barrier layers. These tunneling processes proceed, with a time constant of down to 0.1 ps, very much faster than the transition from the optically active electronic level of the electron quantum film to the hole level of the hole quantum film, which has a relaxation time of approximately 1 ns. As a result, at first glance, the population inversion as a basic prerequisite for laser operation is fulfilled.
Previous considerations regarding the design of the laser material of an interband cascade laser are therefore based on enabling the electrons, or in the complementary picture the holes, to be transported further through the laser medium by resonant tunneling. Taking account of the band tilting as a result of the voltage applied to the laser material in the operating case, the barrier layers, the collector quantum films and the injector quantum films are therefore designed with regard to thickness and semiconductor material such that the subbands of a cascade that are formed by quantization perpendicular to the layer plane are energetically substantially at the same level. This allows the charge carriers to tunnel along the cascade resonantly and thus rapidly. In this case, the charge carriers are transported perpendicular to the layer planes. The charge carriers tunnel since the individual subbands spatially overlap along the transport direction, thus affording a certain probability of transition between the individual subbands.
It is disadvantageous that the charge carriers have no preferred direction whatsoever in the case of a resonant tunneling process. The resonant tunneling process can equally also proceed counter to the transport direction provided and thus refill for example states at the lower laser level, that is to say in the valence band of the hole quantum film, which are then no longer available for generating photons in the next cascade. This is problematic particularly during operation at or above room temperature, since the charge carriers thus gain thermal energy (approximately 26 meV at 300 K) and purported potential barriers that are intended to prevent thermal backfilling can thus also be surmounted increasingly more easily.